Method for forming deposition film and method for producing photovoltaic device

ABSTRACT

A method for forming a deposition film from an aqueous solution by electrochemical reaction includes the steps of: forming the targeted deposition film under primary deposition conditions; replacing at least part of members in contact with the solution or removing deposit on surfaces of the members; and depositing a film under secondary deposition conditions. These steps are performed in that order. Then, the deposition film is formed again under the primary deposition conditions. In the method, the resulting deposition film exhibits desired characteristics even after maintenance of the deposition apparatus.

This application claims priority from Japanese Patent Application No. 2003-408479 filed Dec. 8, 2003, which is hereby incorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to methods for forming a deposition film from an aqueous solution by electrochemical reaction, and particularly to a method for depositing a zinc oxide thin film. In addition, the invention relates to a method for producing a photovoltaic device including the deposition film.

2. Description of the Related Art

Many methods have been proposed for forming a deposition film from a solution by electrochemical reaction. For example, U.S. Pat. No. 5,804,466 (Japanese Patent Laid-Open No. 10-140373) has disclosed a method for depositing a zinc oxide thin film by an electrochemical reaction in a solution. This method can provide zinc oxide thin films more inexpensively than vacuum processes, such as vacuum deposition by resistance heating or electron beaming, sputtering, ion plating, and CVD.

A solution used in such electrochemical reaction has been disclosed in Japanese Patent No. 3273294. The solution contains 0.001 to 0.5 mol/L of zinc ion and 0.001 to 0.5 mol/L of nitrate ion.

U.S. Pat. No. 6,576,112 (Japanese Patent Laid-Open No. 2002-167695) has disclosed a method to provide over a large area a uniform zinc oxide film having a texture exhibiting a high optical confinement effect, preventing the zinc oxide crystals from abnormally growing in spherical or branched form and varying in grain size. In this method, the aqueous solution used for electrochemical deposition contains a polycarboxylic acid in which a carboxyl group is bonded to each of carbon having sp2 hybrid orbital, or an ester of the polycarboxylic acid, in addition to nitrate ions and zinc ions.

These methods have provided zinc oxide film exhibiting superior characteristics.

However, these methods do not produce a deposition film having a satisfactory texture in some cases even though the composition of the solution is supplemented by adding appropriate constituents after repetitive deposition or long-time deposition.

For example, repetition of long-time deposition of a zinc oxide film on a strip substrate or other long substrate results in difficulty controlling the shape and grain size of the zinc oxide crystals constituting the resulting zinc oxide film. Abnormal shape or grain size of the zinc oxide crystals changes optical characteristics of the resulting zinc oxide film. If such a zinc oxide film is used in photovoltaic devices, the resulting devices cannot have uniform characteristics. Furthermore, such a zinc oxide film may negatively affect electrical properties, such as conversion efficiency and deterioration rate, and mechanical characteristics associated with occurrence of film peeling or cracks.

These problems are probably caused by: (1) unnecessary deposit on apparatus members other than the substrate onto which the film is to be deposited; or (2) precipitate produced in the aqueous solution. Specifically, a deposit is formed on members in contact with the solution of the deposition apparatus, such as the wall of the solution bath, an electrode fastener, and a masking plate for preventing deposition on the rear surface. In addition, precipitation may occur in the solution. Furthermore, in the solution, unnecessary impurities for the film deposition, which are contained in supplementarily added zinc nitrite or additives, may increase.

The zinc ions in the solution are consumed for film formation. Accordingly, if the counter electrode for forming the deposition film (zinc oxide film) is made of zinc, the zinc electrode is dissolved in the solution to restore the zinc concentration and the volume of the electrode is reduced. Accordingly, it becomes necessary to replace the electrode.

It is considered that the solution for forming the deposition film can be maintained stable if the amount of unnecessary deposit on the solution bath wall, electrode fastener, masking plate, or other members of the deposition apparatus and precipitate in the solution are within a permissible range. However, once the deposit or precipitate increases beyond the permissible range, the solution probably becomes unstable and a satisfactory texture cannot be provided to the resulting deposition film.

Furthermore, as the thickness of the deposit on the members of the deposition apparatus increases, the deposit becomes liable to peel off. The peeled deposit and the precipitate in the solution can be trapped on the substrate disadvantageously.

In order to solve these problems, appropriate maintenance is performed after the number of times of deposition or the total length of deposition reaches a predetermined level. For example, the electrode, the electrode fastener, the masking plate, and the like are replaced or cleaned, the solution is replaced, or the precipitate is removed from the solution. In film deposition immediately after such maintenance, however, zinc oxide crystals constituting the zinc oxide film do no grow to a desired shape or grain size in some cases even though the composition of the solution is suitable for depositing a targeted zinc oxide film.

SUMMARY OF THE INVENTION

Accordingly, an object of the present invention is to provide a method for forming a deposition film from an aqueous solution by electrochemical reaction and in which a deposition film exhibiting desired characteristics can be produced even after maintenance of the deposition apparatus. Another object of the present invention is to provide a method for producing photovoltaic devices including the deposition film, the devices exhibiting substantially the same characteristics both before and after maintenance.

The inventors of the present invention have conducted intensive research for a method for forming a deposition film having satisfactory characteristics and obtained the following findings.

The inventors have thought that the reason why a solution having an optimum composition for forming a deposition film does not result in a satisfactory texture immediately after maintenance of the deposition apparatus is the variations of the surface conditions of replaced members and the amount of precipitate in the solution.

In the formation of a deposition film from a solution by electrochemical reaction, the solution is prepared to have a composition suitable for the targeted deposition film. The solution is a mixture containing constituents necessary for the targeted deposition film in water. In the method for forming deposition films on substrates, deposit is produced on members in contact with the solution of the deposition apparatus, and precipitate is also produced in the solution. Since the deposit on its members and the precipitate in the solution are at least partially removed by maintenance of the apparatus, reactions occur to reproduce the deposit and precipitate. These reactions probably proceed faster in the early stage. The constituents in the solution are probably consumed not only for the deposition film on the substrate, but also for the deposit on the members and the precipitate in the solution. Consequently, the resulting deposition film on the substrate does not exhibit a satisfactory texture. The rates of the reactions reproducing the deposit on the members and the precipitate in the solution gradually decrease, and the changes in the contents of the constituents in the solution are alleviated so that the solution is substantially in equilibrium. The texture of the deposition film becomes stable only after the solution has come to this state.

However, since deposition over a long period of time makes imbalance the contents of the constituents in the solution, it is necessary to supplementarily add the constituents to the solution in order to stabilize the texture.

The inventors have found that a method for forming a deposition film which can stably and constantly provide a film having a satisfactory texture even after maintenance. In the method, the deposition film is formed under primary conditions, maintenance is performed at a predetermined time interval, subsequently a film is deposited under secondary conditions after the maintenance, and then the deposition film is formed again under the primary conditions.

Accordingly, the present invention is directed to a method for forming a deposition film from an aqueous solution by electrochemical reaction. The method comprises the steps of: forming the deposition film under primary deposition conditions; replacing at least part of members in contact with the solution or removing deposit on surfaces of the members for maintenance; and depositing a film under secondary deposition conditions. The steps are performed in that order.

The method allows the solution to become stable, thereby stably promoting the deposition under the primary conditions to form a targeted deposition film.

Preferably, the method further comprises the step of forming again the deposition film again under the primary deposition conditions after the step of deposition under the secondary conditions.

Thus, the deposition under the primary conditions can proceed in a stable solution even after the maintenance, and the resulting deposition film have the same satisfactory texture before and after the maintenance.

Preferably, the secondary deposition conditions allow a desired film to deposit on the members in contact with the solution.

Thus, the deposition under the primary conditions can stably proceed even after the maintenance.

Preferably, the deposition rate of the secondary deposition conditions is higher than that of the primary deposition conditions.

Thus, the primary deposition conditions can be stabilized in a short time after the maintenance.

Preferably, the deposition under the secondary deposition conditions is performed on a substrate different from the substrate used in the deposition under the primary deposition conditions.

Thus, the primary deposition conditions can be restored by use of an inexpensive substrate.

The present invention is also directed to a method for producing a photovoltaic device, comprising the steps of: forming a zinc oxide film by the foregoing method for forming a deposition film; and forming a silicon-based semiconductor layer after the step of forming the zinc oxide film.

Since this method stably produces photovoltaic devices from the deposition film having a desired texture, the resulting devices exhibit substantially the same characteristics before and after the maintenance.

Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments (with reference to the attached drawings).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an apparatus for depositing zinc oxide from a solution according to the present invention.

FIG. 2 is a schematic sectional view of the layered structure of a photovoltaic device according to the present invention.

FIG. 3 is a schematic sectional view of a continuous electrolytic deposition apparatus for depositing zinc oxide from a solution according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiment will now be described with reference to the drawings.

Method of Electrochemical Deposition from Aqueous Solution

FIG. 1 shows the structure of an apparatus for forming a deposition film from an aqueous solution by electrochemical reaction. The deposition apparatus includes a reactor 101 containing an aqueous solution 102, a conductive substrate 103, an counter electrode 104, a power supply 105, a load resistor 106, a solution outlet 107, a solution inlet 108, a solution intake pipe 109, a solution discharge pipe 110, a circulating pump 111, a sub reactor 112, a heater 113, and a masking plate 114 for preventing film deposition on the rear surface of the substrate 103.

Primary deposition conditions, which are intended for the formation of a targeted deposition film, and secondary deposition conditions include the composition and temperature of the solution 102, the structures and constituents of the conductive substrate 103 and counter electrode 104, the current applied between the conductive substrate 103 and the counter electrode 104, and deposition time.

To at least partially replace members in contact with the solution of the deposition apparatus or remove deposit on the surfaces of the members herein means to replace, for example, the masking plate 114 or a fastener (not shown in the figure) fastening the counter electrode 104 or to remove the films deposited on the surfaces of those members.

The primary deposition conditions and the secondary deposition conditions will be described below.

Secondary Deposition Conditions

The secondary deposition conditions are set for temporary deposition for more stabilizing the state of the deposition apparatus. For example, the secondary conditions are set for forming a film on the members which have been replaced or cleaned to remove deposit. This deposition on the members is intended to produce deposit on the members to such an extent that the texture of the resulting deposition film formed from an aqueous solution for forming a targeted film does not significantly vary from the texture of the targeted deposition film.

The secondary deposition conditions are preferably different from the primary deposition conditions described below, but may be the same.

The deposition rate of the secondary deposition conditions is preferably higher than that of the primary deposition conditions. Thus, the primary deposition conditions can be stabilized in a short time after maintenance.

In order to set the deposition rate of the secondary deposition conditions to be higher than that of the primary deposition conditions, for example, the temperature of the solution or the amount of current is increased, or the composition of the solution or stir speed is varied. For varying the composition of the solution, the ion concentrations or the additive content in the solution for forming the targeted deposition film may be increased. However, the change of the solution composition can involve large change in concentration or other factors when the solution is changed to that of the primary conditions.

The conductive substrate 103 used under the secondary conditions may be different from that under the primary conditions. While the substrate may be treated according to the application for the primary deposition conditions, the substrate used under the second deposition conditions does not require such treatment.

Primary Deposition Conditions

The primary deposition conditions are set as follows for forming a deposition film having a desired texture.

Aqueous Solution 102:

The aqueous solution 102 contains ions for forming a targeted deposition film. For a zinc oxide film, the solution 102 contains at least zinc nitrate and an additive, for example. Deposition of a zinc oxide film will be described below.

Zinc nitrite dissociates into nitrate ions and zinc ions in water. The concentrations of the nitrate ions and the zinc ions are preferably in the range of 0.002 to 3.0 mol/L, more preferably 0.01 to 1.5 mol/L, and still more preferably 0.05 to 0.7 mol/L.

The additive may be a polycarboxylic acid in which a carboxyl group is bonded to each of carbon having sp2 hybrid orbital, or its ester. Preferably, the additive has a —C═C— group whose carbons are each bonded to the carboxyl group or an ester group, or an aromatic ring (benzene ring or heteroaromatic ring) some of whose carbons are bonded to a carboxyl group. Examples of such additives include phthalic acid, isophthalic acid, maleic acid, naphthalic acid, and their esters. The concentration of the polycarboxylic acid or its ester is preferably in the range of 0.5 to 500 μmol/L, more preferably 50 to 500 μmol/L, and still more preferably 150 to 500 μmol/L. Such a concentration of the polycarboxylic acid or its ester contributes to efficient formation of a zinc oxide film having a texture exhibiting a high optical confinement effect suitable as the transparent conductive layer of a photovoltaic device described later.

The solution 102 may further contain saccharose or dextrin. These additives properly control the electrolytic deposition to prevent the zinc oxide film from abnormally growing, so that the resulting film exhibits uniform surface and superior adhesion. Thus, a zinc oxide film having a texture exhibiting a high optical confinement effect can be produced in a high yield. If the solution 102 contains saccharose or dextrin, the saccharose content is preferably in the range of 1 to 500 g/L and more preferably 3 to 100 g/L, and the dextrin content is preferably in the range of 0.01 to 10 g/L and more preferably 0.025 to 1 g/L.

The electrolytic deposition is performed while the composition of the solution is restored by adding its constituents. The solution is prepared so as to have a pH of at least 3 and an electric conductivity of at least 10 mS/cm, and the solution temperature is set at 60° C. or more. Thus, efficient deposition of a uniform zinc oxide film can be achieved without abnormal growth.

Power Supply, Deposition Section:

The targeted deposition film is formed on the conductive substrate 103. The conductive substrate 103 may be conductive itself, or has a structure in which a conductive layer is formed over an insulating substrate. The surface onto which the deposition film is to be formed may be treated according to the application.

The counter electrode 104 is intended for a reaction together with the conductive substrate 103. The counter electrode 104 can be made of any conductive material. A stable material may be suitable from the viewpoint of preventing constituents unnecessary for the formation of the deposition film from being supplied to the solution. The counter electrode 104 may also be made of a material capable of supplying ions necessary for the formation of the deposition film to the solution. For example, the counter electrode 104 may be made of zinc for deposition of a zinc oxide film.

The conductive substrate 103 and the counter electrode 104 are connected to the power supply 105 through the load resistor 106. The current applied between the conductive substrate 103 and the counter electrode 104 is preferably in the range of 0.1 to 100 mA/cm², more preferably 1 to 30 mA/cm², and optimally 4 to 20 mA/cm².

Solution Circulating System:

In order to stir the entire solution, a solution circulating system is used which includes the solution inlet 108, the solution outlet 107, the circulating pump 111, the solution intake pipe 109, and the solution discharge pipe 110.

Photovoltaic Device

FIG. 2 is a schematic sectional view of the layered structure of a photovoltaic device according to the present invention. The device shown in the figure serves as a solar cell and includes a substrate (support) 201, a metal layer (rear reflector) 202, a hexagonal polycrystalline zinc oxide layer (transparent conductive layer) 203, a semiconductor layer 204, a transparent electrode layer 205, and a collector electrode 206. When light enters through the substrate 201, the substrate is transparent and the other layers 202 to 206 are disposed in inverse order on the substrate 201.

The components of the photovoltaic device will be described in detail below.

Substrate

The substrate 201 is made of metal, or resin, glass, ceramic, or the like coated with a conductive material. The surface of the substrate 201 may have a fine texture. The substrate 201 may be transparent so that light enters through the substrate. A substrate 201 made of a flexible material, such as stainless steel or polyimide, can be so long as to be adoptable to continuous deposition.

Metal Layer

The metal layer 202 serves as both an electrode and a reflection layer for reflecting light reaching the substrate 201 to reuse the light in the semiconductor layer 204. The metal layer 202 is formed of, for example, Al, Cu, Ag, Au, and the like by vapor deposition, sputtering, electrolytic deposition, printing, or the like. Also, by giving a texture to the surface of the metal layer 202, the optical path in the semiconductor layer 204 of reflected light is lengthened to increase short-circuit current.

Transparent Conductive Layer

The transparent conductive layer 203 allows incident light and reflected light to diffuse increasingly to lengthen the optical path in the semiconductor layer 204. In addition, the transparent conductive layer 203 prevents the constituent elements of the metal layer 202 from diffusing or migrating into the semiconductor layer 204, and thus prevents the photovoltaic device from shunting. Furthermore, by giving an appropriate resistance to the transparent conductive layer 203, short circuits resulting from defects in the semiconductor layer 204, such as pinholes, can be prevented. Preferably, the surface of the transparent conductive layer 203, as well as the metal layer 202, has a texture effective at exhibiting an optical confinement effect.

The deposition method of the present invention is suitable to form the transparent conductive layer 203. Preferably, a zinc oxide film is formed on the metal layer 202 in advance by sputtering, and then a further zinc oxide film is provided on the previously formed zinc oxide film by the electrolytic deposition. Such a process increases the adhesion between the metal layer 202 and the zinc oxide film or transparent conductive layer 203.

Semiconductor Layer

The semiconductor layer 204 is made of amorphous or microcrystalline silicon, carbon, germanium, or their alloy. The semiconductor layer 204 also contains hydrogen and/or halogen atoms. The preferred content of hydrogen and/or halogen atoms is in the range of 0.1 to 40 atomic percent. The semiconductor layer may further contain oxygen, nitrogen, or other impurities. The concentration of such impurities is preferably 5×10¹⁹ atoms/cm⁻³ or less. For a p-type semiconductor, a Group III element is further added; for an n-type semiconductor, a Group V element.

For a stack cell, it is preferable that the i-type semiconductor layer of a pin junction close to the light incident side has a wide band gap, and that the band gap is reduced as the distance between the light incident side and the pin junction is increased. Preferably, a minimum value of the band gap in the i-type semiconductor layer is on the p-type semiconductor side of the center of the thickness.

For a doped layer at the light incidence side, a crystalline semiconductor having a low light absorption or a semiconductor having a wide band gap is suitable.

For the formation of the semiconductor layer 204, microwave (MW) plasma CVD or high-frequency (RF) CVD is suitably applied.

In this semiconductor deposition, a tip that “i-type layer made of Graded SiGe having a Ge content of 20 to 70 atomic percent” (Japanese Patent Laid-Open No. 6-21494) may be adopted.

Transparent Electrode Layer

The transparent electrode layer 205 can serve as an antireflection layer by appropriately setting the thickness. The transparent electrode layer 205 is formed of ITO, ZnO, In₂O₃, or the like by a method, such as vapor deposition, CVD, spraying, spin-on, or dipping. The transparent electrode layer 205 may further contain a material varying the conductivity.

Collector Electrode

The collector electrode 206 is intended to increase the power collection efficiency. The collector electrode 206 may be formed by depositing a metal in a power collection pattern by sputtering through a mask, by printing a conductive paste or a solder paste, or by bonding metal wires with a conductive paste.

The photovoltaic device may be provided with protective layers on its both surfaces, if necessary. A reinforcement, such as a steel plate, may be used together with the protective layers.

EXAMPLES

Examples according to the present invention will now be described, but the invention is not limited to these examples.

Example 1

A zinc oxide layer (transparent conductive layer) 203 as shown in FIG. 2 was formed with a roll-to-roll type apparatus shown in FIG. 3.

The roll-to-roll type apparatus included a delivery roller 301, a take-up roller 302, a rolled substrate 303, a transport roller 304, a zinc oxide deposition bath 305 containing a solution 306 for depositing the zinc oxide film, an counter electrode 307, a power supply 308, a rinse bath 309 containing water 310, a water shower 311, a drier oven 312, an infrared heater 313, and a masking plate 314 for preventing film deposition on the rear surface of the substrate 303.

In the present example, silver was deposited to a 200 nm thick metal layer (rear reflector) 202 in advance on a rolled SUS4302D support (acting as the substrate 201 shown in FIG. 2) with a DC magnetron sputtering apparatus for the roll-to-roll system. In addition, a zinc oxide film was deposited to a thickness of 100 nm on the metal layer 202 at a substrate temperature of 150° C. and a deposition rate of 5 nm/s with a similar DC magnetron sputtering apparatus for the roll-to-roll system. The resulting support was used as the rolled substrate 303.

Zinc oxide was further deposited to form the zinc oxide layer or transparent conductive layer 203 on the substrate 303 under the below-described primary deposition conditions with the roll-to-roll apparatus shown in FIG. 3.

The substrate 303 was transported to the zinc oxide deposition bath 305 via the transport roller 304. The zinc oxide deposition bath 305 had a sub deposition bath (not shown in the figure). The solution 306 in the zinc oxide deposition bath 305 was prepared by adding 60 g of zinc nitrate hexahydrate into 1 L of water (to be 0.20 mol/L zinc nitrate) and further adding to the solution potassium biphthalate (molecular weight: 204.22) at a concentration of 0.05 mmol/L and 0.3 g of dextrin. The resulting solution 306 was subjected to circulation to be stirred. The total volume of the solution 306 was 1,000 L. The temperature of the bath was maintained at 80° C. and the pH was 4.0 to 5.0. The electric conductivity was 65 mS/cm. The counter electrode 307 was a zinc plate subjected to blasting and was used as the anode, and the substrate 303 was used as the cathode. Thus, electrolytic deposition was performed with the anode counter electrode 307 and the cathode substrate 303 at a current density of 12 mA/cm² (1.2 A/dcm²). The zinc oxide layer was deposited over a length of 10,000 m. On the way of deposition, consumed zinc nitrate and potassium biphthalate in the solution was supplementarily added. The substrate 303 was transported at a speed of 2,000 mm/min, and thus a zinc oxide thin-film of 1.8 μm in thickness, serving as the transparent conductive layer 203, was formed.

The substrate 303 having the zinc oxide layer 203 was transported to the rinse bath 309 and the shower 311 to rinse out the solution remaining on the surface of the substrate 303, and drained with an air knife (not shown in the figure). Then, the substrate 303 was passed through the drier oven 312 to be dried and finally wound around the take-up roller 302.

Then, the masking plate 314, the counter electrode 307, and the counter electrode fastener (not shown in the figure) were replaced for maintenance.

After the maintenance, another deposition under the secondary conditions was performed in the same manner as the deposition under the primary conditions except that the simple SUS4302D support was used as the substrate 303. Thus, zinc oxide was deposited over a length of 100 m directly on the SUS4302D support.

Then, the following deposition was performed under the same conditions as before the maintenance, that is, the deposition under the primary conditions was performed again. Thus, the zinc oxide layer 203 was deposited over a length of 50 m.

Part of the substrate having the zinc oxide layer formed at the point of the completion of the deposition under the primary conditions after the maintenance was cut out for evaluation, and the texture at the surface of the test piece was analyzed with a scanning probe microscope Nanopics produced by Seiko Instruments Inc. The results were shown in the table. In the table, the inclination angle θ, the surface roughness Ra, and the grain size are those of the zinc oxide layer 203 formed at the point of the completion of the deposition under the primary conditions after the maintenance, and are represented by relative values to those (each assumed to be 1) of the zinc oxide layer 203 at the point of the completion of the deposition under the primary conditions before the maintenance.

Then, a semiconductor layer 204 was formed on the zinc oxide layer 203 by CVD. The semiconductor layer 204 included a 10 nm thick n-type microcrystalline silicon sub layer acting as the bottom cell, a 3,000 nm thick non-doped microcrystalline silicon sub layer, and a 30 nm thick p-type microcrystalline silicon sub layer. Subsequently, ITO was deposited to a thickness of 65 nm by sputtering, thus forming a transparent electrode layer 205 serving as an antireflective upper electrode. Finally, silver was deposited in a grid by thermal vapor deposition to form an upper extraction electrode serving as the collector electrode 206, thus completing a photovoltaic device.

The short circuit current density and the conversion efficiency of the photovoltaic device were measured under artificial sunlight. The results were shown in the Table. In the table, the short circuit current density and the conversion efficiency are those of the photovoltaic device made from the substrate having the zinc oxide layer formed at the point of the completion of the deposition under the primary conditions after the maintenance, and are represented by relative values to those (each assumed to be 1) of the photovoltaic device made from the substrate having the zinc oxide layer at the point of the completion of the deposition under the primary conditions before the maintenance.

Example 2

Electrolytic deposition under the secondary conditions was performed with the anode or counter electrode 307 and the cathode or substrate 303 at a current density of 24 mA/cm² (2.4 A/dcm²). Other conditions were the same as in Example 1. Evaluation was also performed in the same manner as in Example 1. The results were shown in the Table.

Example 3

In deposition under the secondary conditions, the substrate was transported at a rate of 1,000 mm/min. Other conditions were the same as in Example 1. Evaluation was also performed in the same manner as in Example 1. The results were shown in the Table.

Example 4

In deposition under the secondary conditions, the solution temperature was set at 90° C. Other conditions were the same as in Example 1. Evaluation was also performed in the same manner as in Example 1. The results were shown in Table 1.

Example 5

For deposition under the secondary deposition, 120 g of zinc nitrate hexahydrate was added into 1 L of water (to be 0.40 mol/L zinc nitrate). Other conditions were the same as in Example 1. Evaluation was also performed in the same manner as in Example 1. The results were shown in the Table.

Comparative Example

The zinc oxide layer or transparent conductive layer 203 was formed in the same manner as in Example 1, except that deposition under the secondary conditions was not performed. Evaluation was also performed in the same manner as in Example 1. The results were shown in the Table. TABLE 1 Surface Short circuit Inclination roughness Grain current Conversion angle θ Ra size density efficiency Example 1 0.98 0.98 1.02 0.99 0.99 Example 2 0.95 0.95 1.05 0.97 0.97 Example 3 0.95 0.95 1.10 0.97 0.97 Example 4 0.95 0.96 1.00 0.96 0.96 Example 5 0.95 0.94 1.05 0.97 0.97 Comparative 0.70 0.70 1.30 0.60 0.60 example 1

The results of Example 1 shown in the Table suggest that a zinc oxide layer having a stable texture can be provided through the process in which deposition under the secondary conditions is performed after maintenance on a substrate different from the substrate used for the deposition under the primary conditions and subsequently the deposition under the primary conditions was performed again. In such a process, the short circuit current and conversion efficiency of the photovoltaic device do not vary much before and after maintenance.

The results of Example 2 suggest that a zinc oxide layer having a stable texture can also be provided through the process in which deposition under the secondary conditions was performed at a higher current density than that of the primary deposition conditions and subsequently the deposition under the primary conditions was performed again. In such a process, the short circuit current and conversion efficiency of the photovoltaic device do not vary much before and after maintenance, and the differences are permissible.

The results of Example 3 suggest that even though the transport speed of the substrate of the secondary conditions is varied, a zinc oxide layer having a stable texture can be provided. In such a process, the short circuit current and conversion efficiency of the photovoltaic device do not vary much before and after maintenance, and the differences are permissible.

The results of Example 4 suggest that even though the solution temperature of the secondary conditions is varied, a zinc oxide layer having a stable texture can be provided. In such a process, the short circuit current and conversion efficiency of the photovoltaic device do not vary much before and after maintenance, and the differences are permissible.

The results of Example 5 suggest that even though the solution concentration of the secondary conditions is varied, a zinc oxide layer having a stable texture can be provided. In such a process, the short circuit current and conversion efficiency of the photovoltaic device do not vary much before and after maintenance, and the differences are permissible.

The results of the comparative example suggest that if deposition under the secondary conditions is not performed, a satisfactory texture cannot be provided to the zinc oxide layer. The short circuit current and the conversion efficiency of the photovoltaic device prepared from such a film are not satisfactory.

While the present invention has been described with reference to what are presently considered to be the preferred embodiments, it is to be understood that the invention is not limited to the disclosed embodiments. On the contrary, the invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 

1. A method for forming a deposition film from an aqueous solution by electrochemical reaction, comprising the steps of: forming the deposition film under primary deposition conditions; performing one of replacing at least part of members in contact with the solution and removing deposit on surfaces of the members; and depositing a film under secondary deposition conditions, wherein the steps are performed in that order.
 2. The method according to claim 1, further comprising the step of forming again the deposition film under the primary deposition conditions after the step of deposition under the secondary conditions.
 3. The method according to claim 1, wherein the secondary deposition conditions allow a desired film to deposit on the members in contact with the solution.
 4. The method according to claim 1, wherein the deposition rate of the secondary deposition conditions is higher than the deposition rate of the primary deposition conditions.
 5. The method according to claim 1, wherein the deposition under the secondary deposition conditions is performed on a substrate different from the substrate used under the primary deposition conditions.
 6. A method for producing a photovoltaic device, comprising the steps of: forming a zinc oxide film by the method as set forth in claim 1; and forming a silicon-based semiconductor layer after the step of forming the zinc oxide film. 